The tissue factor coagulant activity of peripheral blood monocytes was measured in 20 patients on long-term anticoagulant treatment with warfarin and in 21 healthy individuals. Contrary to expectations, tissue factor activity was not reduced in the warfarin-treated group and was, in fact, elevated in several patients. No relationship existed between the daily warfarin dose or the degree of prolongation of the prothrombin time and the monocyte tissue factor activity. These results indicate that monocyte tissue factor activity is not necessarily reduced to below normal levels with warfarin treatment.